Messages in this thread | | | Date | Wed, 16 Apr 2008 10:48:14 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 1/2] Marker probes in futex.c |
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* Arjan van de Ven (arjan@infradead.org) wrote: > On Wed, 16 Apr 2008 10:00:09 -0400 > Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote: > > > > > > > If we want to support NMI context and have the ability to > > > > > instrument preemptable code without too much headache, we must > > > > > insure that every modification will leave the code in a > > > > > "correct" state and that we do not grow the size of any > > > > > reachable instruction. Also, we must insure gcc did not put > > > > > code between these instructions. Modifying non-relocatable > > > > > instructions would also be a pain, since we would have to deal > > > > > with instruction pointer relocation in the breakpoint code when > > > > > the code modification is being done. > > > > > > you also need to make sure no cpu is executing that code ever.. > > > but you already deal with that right? > > > > > > > By "insure that every modification will leave the code in a "correct" > > state", I mean that at any given time before, during or after the code > > modification, if an NMI comes on any CPU and try to run the modified > > code, it should have a valid version of the code to execute. Does it > > make more sense ? > > I understand your words. My concern is that I don't quite understand how you > guarantee that you'll not be executing the code you're modifying. > Just saying "it's consistent before and after" sounds nice but probably isn't > enough to be safe. > Ah, I see. I insert a breakpoint and execute a bypass rather than the code being modified. I only put back the 1st instruction byte after the rest of the instruction has been modified.
> > > > Not only does the compare and jmp need to be consecutive, but the movb > > $0x0,%al also does. I *could* try to detect specific code inserted in > > between, but I really have to make sure I don't get burned by the > > compiler inserting a jmp there. > > I wonder if just sticking in 2 barriers around your code make gcc stop moving stuff too much >
I'm not sure people would like the side-effect for the rest of optimizations, but it should be tried.
Mathieu
-- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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